The invention relates in general to a lithographic plate imaging apparatus and method such as that using an inkjet printhead(s). The apparatus and method utilize a plate transport system designed to transport a group of plates locked together and a wide format inkjet plate printer to image all plates in unison. This technique minimizes interplate skew and misregistration. In a preferred embodiment a carrier type fixture is employed to align the plate media such that the group of plates are positioned so that swaths or raster lines of color separation images are laid down upon the plate media in alignment to a reference edge of the plate group and that skew and misregistration of the images in reference to the edge is minimized.
Modern printing relies heavily on inkjet printing techniques. The term “inkjet” as utilized herein is intended to include all drop-on-demand or continuous inkjet printer systems including, but not limited to, thermal inkjet, piezoelectric, and continuous, all of which are well known in the printing industry. Essentially, an inkjet printer produces images on a receiver medium, such as paper, by ejecting ink droplets onto the receiver medium in an image-wise fashion. The advantages of non-impact, low-noise, low-energy use, and low cost operation, in addition to the capability of the printer to print on plain paper, are largely responsible for the wide acceptance of inkjet printers in the marketplace. This application involves the use of a type of inkjet printer specifically designed to image lithographic printing plates.
Digital computer-aided design of graphical material or text is well known. Electronically derived images of words or graphics presented on a CRT or other type of digital display of a digital computer system can be edited and converted to final hard copy by direct printing with impact printers, laser printers or inkjet printers. This manner of printing or producing hard copy is extremely flexible and useful when printing relatively small print runs. However for larger print runs, printing on printing presses using lithographic plates is still the preferred process. In such a printing process lithographic plates having different color separation images of the desired composite printed image to be formed are each separately imaged and mounted on a printing press in association with respective inking rollers as part of a respective color station. A composite multicolor print is made by moving a sheet, such as of paper, plastic or fabric, to accept respective color ink images from the respective color stations. Typically a respective blanket roller at each color station receives an ink image from a respective lithographic plate, mounted on a printing cylinder or support, which plate selectively receives ink in accordance with an oleophilic image pattern formed thereon.
Fabrication of lithographic plates to form images thereon by inkjet techniques is known. In this regard an inkjet system may be used to apply an oleophilic liquid to form an image on a hydrophilic aluminum surface of a lithographic plate. Additionally other variants include direct deposit of the inkjet image as a hydrophobic image on the plate. Various types of lithographic plate fabrication are described in U.S. 2002/0126189 A1.
A major advantage of lithographic plate fabrication using inkjet is the relatively low cost of producing the imaged lithographic plate. Wide format inkjet printers can be used to inexpensively print specially prepared plates that have nominal dimensions of say 17″×23.″ One of the key requirements for the image on the plate is good registration of the image to the plate and very good registration between images when printed on the press. Small errors in image to plate location can be compensated for in the press, by adjusting the location of the plate, but errors within an image can not be easily compensated. Such errors can come from variation in the printing process in the fast scan direction or the slow scan direction. Variations in the image that are reproduced on all four plates will not cause registration errors, but any differential errors (errors that are different from plate to plate) will result in registration errors when the color separations are applied to paper in the press.
Typical wide format printers have precise motion control of the printhead assembly with placement accuracy as high as {fraction (1/2400)} inch. This is required to prevent banding artifacts and to achieve high productivity levels. Typical wide format printers have relatively simple receiver transport systems. The key requirement is to avoid banding by proper placement of the receiver for each pass of the printhead assembly. The low mass, low speed and short transport distances and the fact that the receiver moves only between printing passes and is held fixed during printing allows for low cost designs such as scuff rollers to transport the receiver. Typical placement accuracy requirements may be only {fraction (1/600)} inch.
Transport of aluminum plates, which may be as thick as 0.012″, that cannot bend easily and may have a lower coefficient of friction than paper causes platesetter designers to invest in much more expensive solutions for plate transport. It would be desirable to maintain the low cost and simple design of a wide format inkjet printer while avoiding differential transport variation that will result in poor image registration on the press.
What is needed then is an economical method to register the images on the lithographic plate during inkjet imaging in an economical fashion. It is also desirable to print the plates as fast as possible, since many applications require fast plate production rates.